The present invention is directed to a system for modulating the capacity of a positive displacement compressor such as a refrigeration and/or air conditioning compressor and more specifically to a system incorporating a valving arrangement for cyclically blocking suction gas flow to the compressor while the compressor is continuously driven.
Capacity modulation is often a desirable feature to incorporate in refrigeration and air conditioning compressors as well as compressors for other applications in order to enable them to better accommodate the wide range of loading to which systems incorporating these compressors may be subjected. Many different approaches have been utilized for providing this capacity modulation feature ranging from controlling of the suction inlet flow such as by throttling to bypassing discharge gas back to the suction inlet and also through various types of cylinder or compression volume porting arrangements.
In multicylinder reciprocating piston type compressors utilizing suction gas control to achieve capacity modulation, it is common to block the flow to one or more but not all of the cylinders. When activated, the capacity of the compressor will be reduced by a percentage nominally equal to the number of cylinders to which suction gas flow has been blocked divided by the total number of cylinders. While such arrangements do provide varying degrees of capacity modulation, the degree of modulation that can be achieved is available only in relatively large discrete steps. For example, in a six cylinder compressor, blocking suction to two cylinders reduces the capacity by ⅓ or 33.3% whereas blocking suction gas flow to four cylinders reduces capacity by ⅔ or 66.6%. This discrete step form of modulation does not allow the system capacity to be matched to the load requirement conditions at all but rather only to very roughly approach the desired capacity resulting in either an excess capacity or deficient capacity. As system conditions will rarely if ever match these gross steps of modulation, the overall operating system efficiency will not be able to be maximized.
Compressors in which discharge gas is recirculated back to suction offer quasi-infinite step modulation of the capacity depending upon the variation and complexity of the bypassing means. However, when discharge gas is recirculated back to suction, the work of compression is lost for that fraction of the gas recirculated thus resulting in reduced system efficiency. Combinations of the aforementioned methods enables substantially quasi-infinite capacity modulation at slightly better efficiency but still fails to provide the ability to closely match the compressor capacity to the load being served.
Other approaches, which can result in selectively disabling the compression process of one or more of the cylinders of a multi-cylinder compressor, such as cylinder porting, stroke altering or clearance volume varying methods result in similar step modulation with a resulting mismatch between load and capacity and additionally suffer from dynamic load unbalance and hence vibration.
The present invention, however, provides a capacity control arrangement which utilizes a pulse width modulation of suction gas flow to the compressor which enables substantially continuous modulation of the capacity from 0% up to 100% or full capacity. Thus the capacity output of the compressor can be exactly matched to system loading at any point in time. Further, in reciprocating piston type compressors, the suction gas flow to each of the cylinders may be controlled simultaneously by this pulse width modulation system so as to eliminate unbalanced operation of the compressor.
The pulse width modulated compressor is driven by a control system that supplies a variable duty cycle control signal based on measured system load. The controller may also regulate the frequency (or cycle time) of the control signal to minimize pressure fluctuations in the refrigerant system. The on time is thus equal to the duty cycle multiplied by the cycle time, where the cycle time is the inverse of the frequency.
The pulse width modulated compressor of the present invention has a number of advantages. Because the instantaneous capacity of the system is easily regulated by variable duty cycle control, an oversized compressor can be used to achieve faster temperature pull down at startup and after defrost without causing short cycling as conventional compressor systems would. Another benefit of the present invention is that the system can respond quickly to sudden changes in condenser temperature or case temperature set points. The controller adjusts capacity in response to disturbances without producing unstable oscillations and without significant overshoot. This capability is of particular advantage in applications involving cooling of display cases in that it allows a much tighter control of temperature within the case thereby enabling the temperature setting to be placed at a higher level without concern that cyclical temperature swings will exceed the temperatures which are considered safe for the particular goods contained therein.
Operating at higher evaporator temperatures reduces the defrost energy required because the system develops frost more slowly at higher temperatures. This also enables the time between defrost cycles to be lengthened.
The pulse width modulated compressor also yields improved oil return. The volume of oil returned to the compressor from the system is dependent in part on the velocity of gas flow to the compressor. In many capacity modulation systems, the return gas flow to the compressor is maintained at a relatively low level thus reducing the return oil flow. However, in the present invention the refrigerant flow pulsates between high capacity and low capacity (e.g. 100% and 0%), thus facilitating increased oil return due to the periods of high velocity gas flow.
Additionally, the pulse width modulated blocked suction system of the present invention is relatively inexpensive to incorporate into a compressor in that only a single valve assembly is required. Further, because of the system's simplicity, it can be easily added to a wide variety of compressor designs including both rotary and scroll as well as reciprocating piston type compressors. Also, because the present invention keeps the driving motor operating while the suction gas flow is modulated, the stress and strain on the motor resulting from periodic start-ups is minimized. Additional improvements in efficiency can be achieved by incorporating a motor control module which may operate to control various operating parameters thereof to enhance its operating efficiency during periods when the motor load is reduced due to unloading of the compressor.
Additional features and benefits of the present invention will become apparent to one skilled in the art from the following detailed description taken in conjunction with the appended drawings.